Systems with rotors and stators such as gas turbines generally require a gap to be left between the rotating part and the stationary part to provide clearance and avoid rubbing. However, providing a gap allows for leakage through the gap. As leakage through such gaps reduces efficiency in gas turbines, there is a general desire to minimise the size of these clearance gaps in gas turbines.
Various factors can play a part in establishing clearance requirements, including the large temperature variations seen in gas turbines. These temperature variations can result in significant amounts of thermal expansion, and this can be further complicated by the use of materials with different coefficients of thermal expansion and by differences in the rate of cooling of different parts.
One example of a place where clearance plays a major part is between the rotor in the turbine of a gas turbine and the stator surrounding the rotor; that is, between the rotor and blades on the one hand and the turbine vane carrier and blades on the other hand. It has been appreciated that improvements could be made that would enable reduction of the clearance in this region.